Cloudflare Durable Objects
Status: Production Ready ✅ Last Updated: 2026-01-21 Dependencies: cloudflare-worker-base (recommended) Latest Versions: wrangler@4.58.0, @cloudflare/workers-types@4.20260109.0 Official Docs: https://developers.cloudflare.com/durable-objects/
Recent Updates (2025):
Oct 2025: WebSocket message size 1 MiB → 32 MiB, Data Studio UI for SQLite DOs (view/edit storage in dashboard) Aug 2025: getByName() API shortcut for named DOs June 2025: @cloudflare/actors library (beta) - recommended SDK with migrations, alarms, Actor class pattern. Note: Beta stability - see active issues before production use (RPC serialization, vitest integration, memory management) May 2025: Python Workers support for Durable Objects April 2025: SQLite GA with 10GB storage (beta → GA, 1GB → 10GB), Free tier access Feb 2025: PRAGMA optimize support, improved error diagnostics with reference IDs Quick Start
Scaffold new DO project:
npm create cloudflare@latest my-durable-app -- --template=cloudflare/durable-objects-template --ts
Or add to existing Worker:
// src/counter.ts - Durable Object class import { DurableObject } from 'cloudflare:workers';
export class Counter extends DurableObject {
async increment(): Promise
// wrangler.jsonc - Configuration { "durable_objects": { "bindings": [{ "name": "COUNTER", "class_name": "Counter" }] }, "migrations": [ { "tag": "v1", "new_sqlite_classes": ["Counter"] } // SQLite backend (10GB limit) ] }
// src/index.ts - Worker import { Counter } from './counter'; export { Counter };
export default {
async fetch(request: Request, env: { COUNTER: DurableObjectNamespaceCount: ${await stub.increment()});
}
};
DO Class Essentials import { DurableObject } from 'cloudflare:workers';
export class MyDO extends DurableObject { constructor(ctx: DurableObjectState, env: Env) { super(ctx, env); // REQUIRED first line
// Load state before requests (optional)
ctx.blockConcurrencyWhile(async () => {
this.value = await ctx.storage.get('key') || defaultValue;
});
}
// RPC methods (recommended)
async myMethod(): Promise
// HTTP fetch handler (optional)
async fetch(request: Request): Promise
export default MyDO; // CRITICAL: Export required
// Worker must export DO class too import { MyDO } from './my-do'; export { MyDO };
Constructor Rules:
✅ Call super(ctx, env) first ✅ Keep minimal - heavy work blocks hibernation wake ✅ Use ctx.blockConcurrencyWhile() for storage initialization ❌ Never setTimeout/setInterval (use alarms) ❌ Don't rely on in-memory state with WebSockets (persist to storage) Storage API
Two backends available:
SQLite (recommended): 10GB storage, SQL queries, atomic operations, PITR KV: 128MB storage, key-value only
Enable SQLite in migrations:
{ "migrations": [{ "tag": "v1", "new_sqlite_classes": ["MyDO"] }] }
SQL API (SQLite backend) export class MyDO extends DurableObject { sql: SqlStorage;
constructor(ctx: DurableObjectState, env: Env) { super(ctx, env); this.sql = ctx.storage.sql;
this.sql.exec(`
CREATE TABLE IF NOT EXISTS messages (id INTEGER PRIMARY KEY, text TEXT, created_at INTEGER);
CREATE INDEX IF NOT EXISTS idx_created ON messages(created_at);
PRAGMA optimize; // Feb 2025: Query performance optimization
`);
}
async addMessage(text: string): Promise
async getMessages(limit = 50): Promise
SQL Methods:
sql.exec(query, ...params) → cursor
cursor.one
Best Practices:
✅ Use ? placeholders for parameterized queries
✅ Create indexes on frequently queried columns
✅ Use PRAGMA optimize after schema changes
✅ Add STRICT keyword to table definitions to enforce type affinity and catch type mismatches early
✅ Convert booleans to integers (0/1) - booleans bind as strings "true"/"false" in SQLite backend
Key-Value API (both backends)
// Single operations
await this.ctx.storage.put('key', value);
const value = await this.ctx.storage.get
// Batch operations await this.ctx.storage.put({ key1: val1, key2: val2 }); const map = await this.ctx.storage.get(['key1', 'key2']); await this.ctx.storage.delete(['key1', 'key2']);
// List and delete all const map = await this.ctx.storage.list({ prefix: 'user:', limit: 100 }); await this.ctx.storage.deleteAll(); // Atomic on SQLite only
// Transactions await this.ctx.storage.transaction(async (txn) => { await txn.put('key1', val1); await txn.put('key2', val2); });
Storage Limits: SQLite 10GB (April 2025 GA) | KV 128MB
WebSocket Hibernation API
Capabilities:
Thousands of WebSocket connections per instance Hibernate when idle (~10s no activity) to save costs Auto wake-up when messages arrive Message size limit: 32 MiB (Oct 2025, up from 1 MiB)
How it works:
Active → handles messages Idle → ~10s no activity Hibernation → in-memory state cleared, WebSockets stay connected Wake → message arrives → constructor runs → handler called
CRITICAL: In-memory state is lost on hibernation. Use serializeAttachment() to persist per-WebSocket metadata.
Hibernation-Safe Pattern
export class ChatRoom extends DurableObject {
sessions: Map
constructor(ctx: DurableObjectState, env: Env) { super(ctx, env); this.sessions = new Map();
// CRITICAL: Restore WebSocket metadata after hibernation
ctx.getWebSockets().forEach((ws) => {
this.sessions.set(ws, ws.deserializeAttachment());
});
}
async fetch(request: Request): Promise
const url = new URL(request.url);
const metadata = { userId: url.searchParams.get('userId'), username: url.searchParams.get('username') };
// CRITICAL: Use ctx.acceptWebSocket(), NOT ws.accept()
this.ctx.acceptWebSocket(server);
server.serializeAttachment(metadata); // Persist across hibernation
this.sessions.set(server, metadata);
return new Response(null, { status: 101, webSocket: client });
}
async webSocketMessage(ws: WebSocket, message: string | ArrayBuffer): Promise
async webSocketClose(ws: WebSocket, code: number, reason: string, wasClean: boolean): Promise
async webSocketError(ws: WebSocket, error: any): Promise
Hibernation Rules:
✅ ctx.acceptWebSocket(ws) - enables hibernation ✅ ws.serializeAttachment(data) - persist metadata ✅ ctx.getWebSockets().forEach() - restore in constructor ✅ Use alarms instead of setTimeout/setInterval ❌ ws.accept() - standard API, no hibernation ❌ setTimeout/setInterval - prevents hibernation ❌ In-progress fetch() - blocks hibernation Alarms API
Schedule DO to wake at future time. Use for: batching, cleanup, reminders, periodic tasks.
export class Batcher extends DurableObject {
async addItem(item: string): Promise
// Schedule alarm if not set
if ((await this.ctx.storage.getAlarm()) === null) {
await this.ctx.storage.setAlarm(Date.now() + 10000); // 10 seconds
}
}
async alarm(info: { retryCount: number; isRetry: boolean }): Promise
const buffer = await this.ctx.storage.get<string[]>('buffer') || [];
await this.processBatch(buffer);
await this.ctx.storage.put('buffer', []);
// Alarm auto-deleted after success
} }
API Methods:
await ctx.storage.setAlarm(Date.now() + 60000) - set alarm (overwrites existing) await ctx.storage.getAlarm() - get timestamp or null await ctx.storage.deleteAlarm() - cancel alarm async alarm(info) - handler called when alarm fires
Behavior:
✅ At-least-once execution, auto-retries (up to 6x, exponential backoff) ✅ Survives hibernation/eviction ✅ Auto-deleted after success ⚠️ One alarm per DO (new alarm overwrites) RPC vs HTTP Fetch
RPC (Recommended): Direct method calls, type-safe, simple
// DO class
export class Counter extends DurableObject {
async increment(): Promise
// Worker calls const stub = env.COUNTER.getByName('my-counter'); const count = await stub.increment(); // Type-safe!
HTTP Fetch: Request/response pattern, required for WebSocket upgrades
// DO class
export class Counter extends DurableObject {
async fetch(request: Request): Promise
// Worker calls const stub = env.COUNTER.getByName('my-counter'); const response = await stub.fetch('https://fake-host/increment', { method: 'POST' }); const data = await response.json();
When to use: RPC for new projects (simpler), HTTP Fetch for WebSocket upgrades or complex routing
Getting DO Stubs
Three ways to get IDs:
idFromName(name) - Consistent routing (same name = same DO) const stub = env.CHAT_ROOM.getByName('room-123'); // Aug 2025: Shortcut for idFromName + get // Use for: chat rooms, user sessions, per-tenant logic, singletons
newUniqueId() - Random unique ID (must store for reuse) const id = env.MY_DO.newUniqueId({ jurisdiction: 'eu' }); // Optional: EU compliance const idString = id.toString(); // Save to KV/D1 for later
idFromString(idString) - Recreate from saved ID const id = env.MY_DO.idFromString(await env.KV.get('session:123')); const stub = env.MY_DO.get(id);
Location hints (best-effort):
const stub = env.MY_DO.get(id, { locationHint: 'enam' }); // wnam, enam, sam, weur, eeur, apac, oc, afr, me
Jurisdiction (strict enforcement):
const id = env.MY_DO.newUniqueId({ jurisdiction: 'eu' }); // Options: 'eu', 'fedramp' // Cannot combine with location hints, higher latency outside jurisdiction
Migrations
Required for: create, rename, delete, transfer DO classes
- Create:
{ "migrations": [{ "tag": "v1", "new_sqlite_classes": ["Counter"] }] } // SQLite 10GB // Or: "new_classes": ["Counter"] // KV 128MB (legacy)
- Rename:
{ "migrations": [ { "tag": "v1", "new_sqlite_classes": ["OldName"] }, { "tag": "v2", "renamed_classes": [{ "from": "OldName", "to": "NewName" }] } ]}
- Delete:
{ "migrations": [ { "tag": "v1", "new_sqlite_classes": ["Counter"] }, { "tag": "v2", "deleted_classes": ["Counter"] } // Immediate deletion, cannot undo ]}
- Transfer:
{ "migrations": [{ "tag": "v1", "transferred_classes": [ { "from": "OldClass", "from_script": "old-worker", "to": "NewClass" } ]}]}
Migration Rules:
❌ Atomic (all instances migrate at once, no gradual rollout) ❌ Tags are unique and append-only ❌ Cannot enable SQLite on existing KV-backed DOs ✅ Code changes don't need migrations (only schema changes) ✅ Class names globally unique per account Common Patterns
Rate Limiting:
async checkLimit(userId: string, limit: number, window: number): Promiserate:${userId})) || [];
const valid = requests.filter(t => Date.now() - t < window);
if (valid.length >= limit) return false;
valid.push(Date.now());
await this.ctx.storage.put(rate:${userId}, valid);
return true;
}
Session Management with TTL:
async set(key: string, value: any, ttl?: number): Promise
async alarm(): Promise
Leader Election:
async electLeader(workerId: string): Promise
Multi-DO Coordination:
// Coordinator delegates to child DOs
const gameRoom = env.GAME_ROOM.getByName(gameId);
await gameRoom.initialize();
await this.ctx.storage.put(game:${gameId}, { created: Date.now() });
Critical Rules Always Do
✅ Export DO class from Worker
export class MyDO extends DurableObject { } export default MyDO; // Required
✅ Call super(ctx, env) in constructor
constructor(ctx: DurableObjectState, env: Env) { super(ctx, env); // Required first line }
✅ Use new_sqlite_classes for new DOs
{ "tag": "v1", "new_sqlite_classes": ["MyDO"] }
✅ Use ctx.acceptWebSocket() for hibernation
this.ctx.acceptWebSocket(server); // Enables hibernation
✅ Persist critical state to storage (not just memory)
await this.ctx.storage.put('important', value);
✅ Use alarms instead of setTimeout/setInterval
await this.ctx.storage.setAlarm(Date.now() + 60000);
✅ Use parameterized SQL queries
this.sql.exec('SELECT * FROM table WHERE id = ?', id);
✅ Minimize constructor work
constructor(ctx, env) { super(ctx, env); // Minimal initialization only ctx.blockConcurrencyWhile(async () => { // Load from storage }); }
Never Do
❌ Create DO without migration
// Missing migrations array = error
❌ Forget to export DO class
class MyDO extends DurableObject { } // Missing: export default MyDO;
❌ Use setTimeout or setInterval
setTimeout(() => {}, 1000); // Prevents hibernation
❌ Rely only on in-memory state with WebSockets
// ❌ WRONG: this.sessions will be lost on hibernation // ✅ CORRECT: Use serializeAttachment()
❌ Deploy migrations gradually
Migrations are atomic - cannot use gradual rollout
❌ Enable SQLite on existing KV-backed DO
// Not supported - must create new DO class instead
❌ Use standard WebSocket API expecting hibernation
ws.accept(); // ❌ No hibernation this.ctx.acceptWebSocket(ws); // ✅ Hibernation enabled
❌ Assume location hints are guaranteed
// Location hints are best-effort only
Known Issues Prevention
This skill prevents 20 documented issues:
Issue #1: Class Not Exported
Error: "binding not found" or "Class X not found" Source: https://developers.cloudflare.com/durable-objects/get-started/ Why It Happens: DO class not exported from Worker Prevention:
export class MyDO extends DurableObject { } export default MyDO; // ← Required
Issue #2: Missing Migration
Error: "migrations required" or "no migration found for class" Source: https://developers.cloudflare.com/durable-objects/reference/durable-objects-migrations/ Why It Happens: Created DO class without migration entry Prevention: Always add migration when creating new DO class
{ "migrations": [ { "tag": "v1", "new_sqlite_classes": ["MyDO"] } ] }
Issue #3: Wrong Migration Type (KV vs SQLite)
Error: Schema errors, storage API mismatch Source: https://developers.cloudflare.com/durable-objects/api/sqlite-storage-api/ Why It Happens: Used new_classes instead of new_sqlite_classes Prevention: Use new_sqlite_classes for SQLite backend (recommended)
Issue #4: Constructor Overhead Blocks Hibernation Wake
Error: Slow hibernation wake-up times Source: https://developers.cloudflare.com/durable-objects/best-practices/access-durable-objects-storage/ Why It Happens: Heavy work in constructor Prevention: Minimize constructor, use blockConcurrencyWhile()
constructor(ctx, env) { super(ctx, env); ctx.blockConcurrencyWhile(async () => { // Load from storage }); }
Issue #5: setTimeout Breaks Hibernation
Error: DO never hibernates, high duration charges Source: https://developers.cloudflare.com/durable-objects/concepts/durable-object-lifecycle/ Why It Happens: setTimeout/setInterval prevents hibernation Prevention: Use alarms API instead
// ❌ WRONG setTimeout(() => {}, 1000);
// ✅ CORRECT await this.ctx.storage.setAlarm(Date.now() + 1000);
Issue #6: In-Memory State Lost on Hibernation
Error: WebSocket metadata lost, state reset unexpectedly Source: https://developers.cloudflare.com/durable-objects/best-practices/websockets/ Why It Happens: Relied on in-memory state that's cleared on hibernation Prevention: Use serializeAttachment() for WebSocket metadata
ws.serializeAttachment({ userId, username });
// Restore in constructor ctx.getWebSockets().forEach(ws => { const metadata = ws.deserializeAttachment(); this.sessions.set(ws, metadata); });
Issue #7: Outgoing WebSocket Cannot Hibernate
Error: High charges despite hibernation API Source: Cloudflare Docs | GitHub Issue #4864 Why It Happens: Durable Objects maintaining persistent connections to external WebSocket services using new WebSocket('url') cannot hibernate and remain pinned in memory indefinitely Use Cases Affected:
Real-time database subscriptions (Supabase, Firebase) Message brokers (Redis Streams, Apache Kafka) WebSocket connections to external real-time services Inter-service communication Prevention: Only use hibernation for server-side (incoming) WebSockets via ctx.acceptWebSocket(). Outgoing WebSocket connections created with new WebSocket(url) prevent hibernation. Redesign architecture to avoid outgoing WebSocket connections from Durable Objects if hibernation is required. Issue #8: Global Uniqueness Confusion
Error: Unexpected DO class name conflicts Source: https://developers.cloudflare.com/durable-objects/platform/known-issues/#global-uniqueness Why It Happens: DO class names are globally unique per account Prevention: Understand DO class names are shared across all Workers in account
Issue #9: deleteAll Issues
Error: Storage not fully deleted, billing continues; or internal error in alarm handler Source: KV Storage API | GitHub Issue #2993 Why It Happens:
KV backend deleteAll() can fail partially (not atomic)
SQLite: calling deleteAll() in alarm handler causes internal error and retry loop (fixed in runtime) Prevention:
Use SQLite backend for atomic deleteAll
In alarm handlers, call deleteAlarm() BEFORE deleteAll():
async alarm(info: { retryCount: number }): Promise
Issue #10: Binding Name Mismatch
Error: Runtime error accessing DO binding Source: https://developers.cloudflare.com/durable-objects/get-started/ Why It Happens: Binding name in wrangler.jsonc doesn't match code Prevention: Ensure consistency
{ "bindings": [{ "name": "MY_DO", "class_name": "MyDO" }] }
env.MY_DO.getByName('instance'); // Must match binding name
Issue #11: State Size Exceeded
Error: "state limit exceeded" or storage errors Source: https://developers.cloudflare.com/durable-objects/platform/pricing/ Why It Happens: Exceeded 1GB (SQLite) or 128MB (KV) limit Prevention: Monitor storage size, implement cleanup with alarms
Issue #12: Migration Not Atomic
Error: Gradual deployment blocked Source: https://developers.cloudflare.com/workers/configuration/versions-and-deployments/gradual-deployments/ Why It Happens: Tried to use gradual rollout with migrations Prevention: Migrations deploy atomically across all instances
Issue #13: Location Hint Ignored
Error: DO created in wrong region Source: https://developers.cloudflare.com/durable-objects/reference/data-location/ Why It Happens: Location hints are best-effort, not guaranteed Prevention: Use jurisdiction for strict requirements
Issue #14: Alarm Retry Failures
Error: Tasks lost after alarm failures Source: https://developers.cloudflare.com/durable-objects/api/alarms/ Why It Happens: Alarm handler throws errors repeatedly Prevention: Implement idempotent alarm handlers
async alarm(info: { retryCount: number }): Promise
Issue #15: Fetch Blocks Hibernation
Error: DO never hibernates despite using hibernation API Source: https://developers.cloudflare.com/durable-objects/concepts/durable-object-lifecycle/ Why It Happens: In-progress fetch() requests prevent hibernation Prevention: Ensure all async I/O completes before idle period
Issue #16: Boolean Values Bind as Strings in SQLite
Error: Boolean columns contain strings "true"/"false" instead of integers 0/1; SQL queries with boolean comparisons fail Source: GitHub Issue #9964 Why It Happens: JavaScript boolean values are serialized as strings in Durable Objects SQLite (inconsistent with D1 behavior) Prevention: Manually convert booleans to integers and use STRICT tables
// Convert booleans to integers this.sql.exec('INSERT INTO test (bool_col) VALUES (?)', value ? 1 : 0);
// Use STRICT tables to catch type mismatches early
this.sql.exec(CREATE TABLE IF NOT EXISTS test (
id INTEGER PRIMARY KEY,
bool_col INTEGER NOT NULL
) STRICT;);
Issue #17: RPC ReadableStream Cancel Logs False Network Errors
Error: Wrangler dev logs show "Network connection lost" when canceling ReadableStream from RPC, despite correct cancellation Source: GitHub Issue #11071 Why It Happens: Canceling ReadableStream returned from Durable Object via RPC triggers misleading error logs in Wrangler dev (presentation issue, not runtime bug) Prevention: No workaround available. The cancellation works correctly - ignore the false error logs in Wrangler dev. Issue does not appear in production or workerd-only setup.
Issue #18: blockConcurrencyWhile Does Not Block in Local Dev (Fixed)
Error: Constructor's blockConcurrencyWhile doesn't block requests in local dev, causing race conditions hidden during development Source: GitHub Issue #8686 Why It Happens: Bug in older @cloudflare/vite-plugin and wrangler versions Prevention: Upgrade to @cloudflare/vite-plugin v1.3.1+ and wrangler v4.18.0+ where this is fixed
Issue #19: RPC Between Multiple wrangler dev Sessions Not Supported
Error: "Cannot access MyDurableObject#myMethod as Durable Object RPC is not yet supported between multiple wrangler dev sessions" Source: GitHub Issue #11944 Why It Happens: Accessing a Durable Object over RPC from multiple wrangler dev instances (e.g., separate Workers in monorepo) is not yet supported in local dev Prevention: Use wrangler dev -c config1 -c config2 to run multiple workers in single session, or use HTTP fetch instead of RPC for cross-worker DO communication during local development
Issue #20: state.id.name Undefined in Constructor (vitest Regression)
Error: DurableObjectState.id.name is undefined in constructor when using @cloudflare/vitest-pool-workers 0.8.71 Source: GitHub Issue #11580 Why It Happens: Regression in vitest-pool-workers 0.8.71 (worked in 0.8.38) Prevention: Downgrade to @cloudflare/vitest-pool-workers@0.8.38 or upgrade to later version where this is fixed
Configuration & Types
wrangler.jsonc:
{ "compatibility_date": "2025-11-23", "durable_objects": { "bindings": [{ "name": "COUNTER", "class_name": "Counter" }] }, "migrations": [ { "tag": "v1", "new_sqlite_classes": ["Counter"] }, { "tag": "v2", "renamed_classes": [{ "from": "Counter", "to": "CounterV2" }] } ] }
TypeScript:
import { DurableObject, DurableObjectState, DurableObjectNamespace } from 'cloudflare:workers';
interface Env { MY_DO: DurableObjectNamespace
export class MyDurableObject extends DurableObject
Official Documentation Durable Objects: https://developers.cloudflare.com/durable-objects/ State API (SQL): https://developers.cloudflare.com/durable-objects/api/sqlite-storage-api/ WebSocket Hibernation: https://developers.cloudflare.com/durable-objects/best-practices/websockets/ Alarms API: https://developers.cloudflare.com/durable-objects/api/alarms/ Migrations: https://developers.cloudflare.com/durable-objects/reference/durable-objects-migrations/ Best Practices: https://developers.cloudflare.com/durable-objects/best-practices/ Pricing: https://developers.cloudflare.com/durable-objects/platform/pricing/
Questions? Issues?
Check references/top-errors.md for common problems Review templates/ for working examples Consult official docs: https://developers.cloudflare.com/durable-objects/ Verify migrations configuration carefully
Last verified: 2026-01-21 | Skill version: 3.1.0 | Changes: Added 5 new issues (boolean binding, RPC stream cancel, blockConcurrencyWhile local dev, RPC multi-session, vitest regression), expanded Issue #7 (outgoing WebSocket use cases) and Issue #9 (deleteAll alarm interaction), added STRICT tables best practice, updated @cloudflare/actors beta warning